Extrusion grade polyethylene iso/terephthalate copolymer

ABSTRACT

A process is disclosed for producing a modified polyethylene iso-terephthalate copolymer which may be processed into plastic containers using conventional extrusion blow molding equipment. The copolymer is modified by the incorporation into a polyethylene iso/terephthalate prepolymer of a chain branching agent and a chain terminating agent. The prepolymer is subsequently solid-state polymerized to form a polyethylene iso/terephthalate copolymer having high zero shear rate melt viscosity and shear sensitivity. This modified copolymer is transparent when extrusion blow molded into a shaped article. The modified copolymer may be extrusion blow molded in intermittent and continuous processes.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 894,674, filed Apr. 10, 1978 now U.S. Pat. No. 4,161,579.

BACKGROUND OF THE INVENTION

Containers used is the cosmetics, food and drug industries have in the past generally been constructed of glass or certain plastic resins, e.g., polyvinyl chloride, polypropylene, polycarbonate or polyacrylonitrile resins. In many instances, these containers have proven to be unsatisfactory or undesirable from the standpoint of cost or biodegradability or for various other reasons. For example, it has been discovered that containers, e.g., bottles, constructed of polyacrylonitrile resin give off noxious fumes when incinerated, causing severe environmental pollution problems. Recently, the use of polyacrylontrile resin is such bottle construction has been stopped completely. Other containers, such as glass, have obvious drawbacks, being readily breakable, bulky to transport and expensive to manufacture, requiring excessive energy consumption.

One material which has been investigated as a possible source of improved construction material for containers in the cosmetics, food and drug industries has been polyethylene terephthalate (PET). However, because of the fluid melt characteristics of PET, new, expensive and specialized equipment has had to be developed to process PET into containers and bottles. This equipment is generally based on processes which involve injection molding of a cylindrical or tubular preform which is subsequently reheated and simultaneously stretched and blown in another machine to form a container or bottle. See, e.g., U.S. Pat. No. 3,778,214.

Attempts to modify PET for use in injection molding processes include, e.g., U.S. Pat. Nos. 3,692,744 and 3,673,139. These patents deal with the incorporation of a chain branching agent, alone or in combination with a crystallization-promoting agent into PET in an effort to produce a composition which may be injection molded. The drawbacks of such injection molding processes have been already pointed out. While the later of these patents suggests that PET modified with a chain branching agent and a crystallization promoting agent may be suitable for extrusion molding applications, such suitability is expressly limited to the production of semi-final goods such as rods, sheets and embossed sheets. The process is not disclosed as applicable to extrusion blow molding of a parison to form hollow structures such as containers or bottles. In fact, it had not been possible until the invention disclosed in applicants' parent, co-pending application, U.S. application Ser. No. 894,674, to successfully mold containers and bottles of PET which would serve as acceptable cosmetics, food or drug containers by the use of conventional extrusion blow molding equipment common in these industries.

In the past, in other areas of application unrelated to the extrusion blow molding of containers and bottles, attempts have been made to modify various polyesters, some including PET, by the incorporation of a chain branching agent or a chain terminating agent or both.

The attempts have been concerned with rendering polyesters suitable for use, e.g., as electrical or thermal insulating materials, coating compositions, or fiber and filament-forming materials with increased dye receptivity. Representative examples of such attempts include U.S. Pat. Nos. 2,606,161; 2,936,296; 2,951,827; 3,033,824; 3,039,979; 3,055,867; 3,223,666; and 3,546,180.

Applicants, in their parent, co-pending application Ser. No. 894,674, solved these problems of the prior art and discovered that a high melt-strength, gel-free modified polyethylene terephthalate suitable for use in conventional extrusion blow molding applications could be obtained by the incorporation of specific amounts of selected chain branching and chain terminating agents in combination. It has been observed, however, that when polyethylene terephthalate modified in this manner is extrusion blow molded by conventional techniques, the resulting product may contain opaque portions therein. In particular, when such a modified polyethylene terephthalate is extrusion blow molded to form a container or bottle, the thicker neck section of the bottle may be rendered opaque. For many applications in the cosmetics, food and drug industries, it is aesthetically desirable or necessary to employ only containers which are transparent throughout, i.e., containers having no opaque section therein.

It was not until the present invention that it was discovered that a high melt-strength, gel-free polyethylene iso/terephthalate copolymer could be obtained by incorporation of specific amounts of selected chain branching agents and chain terminating agents in combination, which copolymer is capable of being extrusion blow molded by conventional techniques to form objects which are transparent throughout, containing no opaque sections.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, a general object of the present invention is to avoid or substantially alleviate the above problems of the prior art.

A more specific object of the present invention is to provide a process for preparing improved polyethylene iso/terephthalate copolymers of increased shear sensitivity.

Another object of the present invention is to provide a process for preparing improved polyethylene iso/terephthalate copolymers which may be extrusion blow molded to form shaped articles which are transparent throughout.

A further object of the present invention is to provide a process for preparing improved polyethylene iso/terephthalate copolymers suitable for fabrication into shaped articles by techniques requiring self-supporting, high melt-strength extrudates whereby the shaped articles are transparent throughout.

A further object of the present invention is to provide a process for preparing improved polyethylene iso/terephalate copolymers which contain substantially no gels.

Still another object of the present invention is to provide the improved polyethylene iso/terephthalate copolymers prepared by these processes.

Yet another object of the present invention is to provide improved processes for extrusion blow molding of containers which are transparent throughout utilizing these improved polyethylene iso/terephthalate copolymers.

Other objects and advantages of this invention will become apparent from the following summary and description of the preferred embodiments set out below.

In one aspect, the present invention provides a high melt-strength, gel-free, branched polyethylene iso/terephthalate copolymer, capable of being extrusion blow molded to form shaped articles which are transparent throughout. This copolymer comprises a polyethylene iso/terephthalate copolymer which has been modified by the incorporation of from about 0.025 to about 1.5 mol percent of a chain branching agent and from about 0.25 to about 10 equivalents based on the number of moles of chain branching agent of a chain terminating agent. The chain branching agent may be one or more members selected from the group consisting of pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is a integer from 3 to 6; compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; compounds having the formula: ##STR1## wherein m is an integer from 1 to 6 and m' is an integer from 3 to 6; and compounds having the formula: ##STR2## wherein n is an integer from 3 to 4.

The chain terminating agent has a boiling point of above about 200° C., is stable at temperatures of up to about 305° C. and is selected from one or more members of the group consisting of aromatic monocarboxylic acids having the formula: ##STR3## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; monofunctional esters having the formula: ##STR4## wherein R³ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR5## wherein Y is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.q CH.sub.3-q COOH

where q is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁴ is a member selected from the group consisting of benzyl radicals and aliphatic radicals having from 1 to 18 carbon atoms; and which are the reaction product of an alcohol and a monocarboxylic acid; and difunctional esters having the formula: ##STR6## wherein R⁵ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR7## wherein Z is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.r CH.sub.3-r COOH

where r is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁶ is a member selected from the group consisting of saturated aliphatic radicals having from 2 to 18 carbon atoms and saturated alicyclic radicals having from 5 to 18 carbon atoms; and which are the reaction product of a diol and a monocarboxylic acid. The modified polyethylene iso/terephthalate copolymer has a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise and a melt viscosity at a shear rate of about 10³ seconds ⁻¹ and at a temperature of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise.

In another aspect, the present invention provides a high melt-strength, gel-free, branched polyethylene iso/terephthalate copolymer, capable of being extrusion blow molded to form shaped articles which are transparent throughout, which copolymer has been modified by the incorporation of from about 0.025 to about 1.5 mol percent of a single composition which acts as both a chain branching agent and a chain terminating agent. This composition is a polyfunctional ester having a boiling point of above about 200° C. and being stable at temperatures of up to about 305° C. and having the formula: ##STR8## wherein a is an integer from 3 to 6; and which is derived by the reaction of a chain branching agent selected from the group consisting of pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6; compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; and compounds having the formula: ##STR9## wherein m is an integer from 1 to 6 and m' is an integer from 3 to 6 with a chain terminating agent selected from one or more members of the group consisting of aromatic monocarboxylic acids having the formula: ##STR10## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms; a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is a integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms.

In this aspect of the invention, the modified polyethylene iso/terephthalate copolymer also has a zero shear rate melt viscosity at a temperature in the range of about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise.

In another aspect, the present invention provides for a process of preparing a high melt-strength, gel-free branched polyethylene iso/terephthalate copolymer, capable of being extrusion blow molded to form shaped articles which are transparent throughout, by first forming a polyethylene iso/terephthalate prepolymer which has been modified by the incorporation of from about 0.025 to about 1.5 mol percent of one or more chain branching agents as described above in connection with the first-mentioned aspect of the present invention and from about 0.25 to about 10 equivalents based on the number of moles of chain branching agent of a chain terminating agent, also as described above in connection with the first-mentioned aspect of the present invention.

The prepolymer which is formed has an inherent viscosity of from about 0.3 to about 1.0 and is subsequently solid-state copolymerized to form a polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise and an inherent viscosity greater than the inherent viscosity of the prepolymer.

In yet another aspect, the present invention provides for a process of preparing a high melt-strength, gel-free, branched polyethylene iso/terephthalate copolymer capable of being extrusion blow molded to form shaped articles which are transparent throughout, by first forming a polyethylene iso/terephthalate prepolymer which has been modified by the incorporation of from about 0.025 to about 1.5 mol percent of a single composition which acts as both a chain branching agent and a chain terminating agent. This composition is the polyfunctional ester which is described above in connection with the second-mentioned aspect of the present invention. The prepolymer which is formed has an inherent viscosity of from about 0.3 to about 1.0 and is subsequently solid-state copolymerized to form a polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to 300° C. of from about 10⁵ to about 10⁶ poise, a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise and an inherent viscosity greater than the inherent viscosity of the prepolymer.

In a further aspect, the present invention provides a process for blow molding a modified polyethylene iso/terephthalate copolymer into containers on an extrusion blow molding machine. The machine comprises an extruder having a die gap at one end thereof and an operable mold positioned adjacent the die gap. The copolymer is extruded through the die gap to form a parison, and a mold is closed about the parison. Gas is introduced into the parison to expand it into the shape of the internal surface of the mold. The copolymer has a zero shear rate melt viscosity at a temperature of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise, a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise and inherent viscosity greater than the inherent viscosity of the prepolymer. The shaped containers prepared by this process are transparent throughout.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated above, the present invention is concerned with the formation and use of a modified polyethylene iso/terephthalate copolymer suitable for extrusion blow molding applications.

To perform a successful extrusion blow molding operation with polyethylene terephthalate (PET)-type polymers, the molten polymer must form into a stable parison for a time long enough to permit a mold to enclose the parison. If the molten polymer does not possess this sufficient "melt strength" or melt viscosity, the parison will tend to elongate or draw under its own weight and either not be blow moldable or result in a blow molded article which has a non-uniform wall thickness, low surface gloss, poorly defined sample shape, and a large number of pitmarks.

Melt strength (MS) may be defined as follows:

ti MS=T₁ /T₂

wherein T₁ equals the time necessary to extrude the first three inches of a six-inch continuous strand of molten polyester, and T₂ equals the time necessary to extrude the second three inches of the six-inch continuous strand. Melt strength may be measured by extruding the six inch strand through a constant drive melt index apparatus with a constant drive piston (0.25 inches/minute) at a temperature high enough to keep the polyester molten (generally about 285° C.). A melt strength value of from about 1.0 to about 2.0 is desirable when the material is to be used in extrusion blow molding applications. Ideally, a value of 1.0 is desired since this would mean that the second three-inch portion extruded at the same rate as the first.

For polyesters with poor or low melt strength, the second segment is extruded much more rapidly than the first segment, resulting in a T₁ /T₂ ratio significantly greater than 1.0.

Thus, polyesters having poor or very low melt strengths have rather large values of T₁ /T₂. By saying that certain polyesters have "no melt strength" is meant that the second segment of the six-inch strand is extruded so rapidly that T₂ cannot be measured.

The term "high melt strength" polyesters refers to polyesters having a ratio of T₁ /T₂ approaching the ideal value of 1.0, and the terms "poor" or "low melt strength" polyester refer to polyesters having comparatively large T₁ /T₂ ratios. Polyesters having "no melt strength" have so small a T₂ value that the melt strength cannot be measured.

The terms "polyester" as used herein is any high molecular weight synthetic polymer composed of at least about 85% by weight of an ester of a dihydric alcohol and terephthalic acid (p-HOOC--C₆ H₄ --COOH).

The term "parison" as used herein refers to a molten, extruded preform, generally in tubular or similar form.

The improved polyethylene iso/terephthalate copolymers have melt strengths of generally less than about 2.0, typically less than about 1.5 and preferably less than about 1.2 at 285° C.

The improved melt strength copolymers of the present invention also have increased inherent viscosities. The "inherent viscosity" (I.V.) of the copolymers of the present invention may be conveniently determined by the equation:

    I.V.=1n N.sub.r /c

wherein N_(r) is the "relative viscosity" obtained by dividing the viscosity of a dilute solution of the copolymer by the viscosity of the solvent employed (measured at the same temperature), and c is the copolymer concentration in the solution, expressed in grams/100 ml of solvent. Improved copolymer inherent viscosity is generally determined in a 0.2 weight percent solution of trifluoroacetic acid/methylene chloride (1:1 v/v). Where solubility of samples is a problem, inherent viscosity of samples is determined in a 0.2 weight percent solution of hexafluoroisopropanol (HFIP). Determinations are made at a temperature of about 25° C. The inherent viscosity of the improved copolymers of the present invention is above about 1.0 and preferably from about 1.4 to about 1.6 in a 0.2 weight percent solution of trifluoroacetic acid/methylene chloride (1:1 v/v).

Inherent viscosities for prepolymer solutions are measured in an I.V. solution of 0.2 weight percent orthochlorophenol at about 120° C. or in an equal volume mixture of trifluoroacetic acid and methylene chloride at about 25° C., and preferably range from about 0.3 to about 1.0.

Besides having sufficient melt viscosity or "melt strength" the copolymers, to be successfully used in extrusion blow molding applications, should also possess sufficient die swell, i.e., the molten copolymer should expand as it is released from the die. This die swell is important for extrusion blow molding applications since (a) the larger the diameter of the extruded copolymer, the easier it is for air to be blown into the parison, and (b) the greater than die swell the greater the expansion of the molten copolymer to fit the particular mold.

The key to this invention is the preparation of a modified copolymer having a sufficient shear sensitivity to render the copolymer acceptable for use in extrusion blow molding operations, while maintaining a polymer composition which will result in extrusion blow molded articles which are transparent throughout. "Shear sensitivity" is a measure of the change in viscosity of copolymer as a function of shear rate. At zero shear rate or at a shear rate which is sufficiently low such that viscosity is independent of shear rate, viscosities generally in the range of about 10⁵ to about 10⁶ poise are considered acceptable for extrusion blow molding applications. To enable the copolymer to move through an extrusion die at reasonable temperatures and pressures and to allow the extruded parison to hang prior to blowing without deformation under its own weight, the melt viscosity must be shear rate dependent. That is, as the shear rate is increased from zero, the melt viscosity should correspondingly decrease, e.g., the viscosity should be generally in the range of about 10³ to about 10⁴ poise at a shear rate of about 10.sup. 3 seconds⁻¹, so that the resin will readily flow through the extrusion head. When application of shear stress is again lowered or ceases, the melt viscosity should again rise to its initial level. Shear rates are measured at a temperature in the range of from about 265° C. to about 300° C.

When extrusion blow molded, the copolymers of the present invention result in shaped articles which are transparent throughout. As used herein, the term "transparent" means having the property of transmitting light without appreciable scattering so that bodies lying beyond are substantially entirely visible. When a shaped article is "transparent throughout", the article is transparent regardless of the relative thickness of the various portions of the article. Further, the term "opaque" as used herein means the property of not being pervious to light.

Linear PET has relatively low molecular weight and a narrow molecular weight distribution. This combination of characteristics results in a low melt strength polymer having a low shear sensitivity which is unacceptable for extrusion blow molding applications. The incorporation of particular chain branching agents and chain stopping agents avoids these problems. Both the incorporation of the chain branching agent and solid state polymerization are believed to increase the molecular weight of a solid-state polymerized PET polymer and broaden the molecular weight distribution thereof. However, when such a PET polymer is extrusion blow molded by conventional techniques, the resulting product may contain unacceptable opaque portions therein.

The present invention avoids this problem by the formation of a polyethylene iso/terephthalate copolymer. The isophthalic portion of the copolymer is obtained by incorporating as a starting material either isophthalic acid or dimethyl isophthalate, as described in detail below.

Isophthalic acid or dimethyl isophthalate is substituted for a portion of the terephthalic acid or dimethyl terephthalate which would be used to produce a pure polyethylene terephthalate polymer, the substitution being generally in an amount sufficient to produce a copolymer which when extrusion blow molded forms shaped articles which are transparent throughout, preferably in amounts up to about 15.0% by weight, and most preferably in amounts from about 5.0% to about 15.0% by weight based on the amount of terephthalic acid or dimethyl terephthalate required to produce a corresponding polyethylene terephthalate polymer. Thus, the weight ratio of terephthalic acid structural units to isophthalic acid structural units in the improved copolymer is preferably from about 85:15 and above, and most preferably from about 85:15 to about 95:5.

It is believed that the presence of the isophthalic acid structural units in specifically defined amounts decreases or depresses the melting point of the copolymer when compared to a corresponding PET polymer modified with a chain branching agent and chain terminating agent as described herein. This allows solid-state polymerization to be carried out at lower temperatures, e.g., preferably in the range of 190° C.-210° C. as compared to 220° C., the preferred solid-state polymerization temperature for the corresponding PET polymer. Further, it is believed that the crystallization rate and crystallization temperature of the polyethylene iso/terephthalate copolymer are also depressed, which would allow the copolymer to be more easily quenched during the extrusion blow molding operation, thereby eliminating the opaqueness in the thicker portions of shaped containers or other articles formed by extrusion blow molding the copolymer.

It is necessary, however, to avoid the presence or incorporation of excessive amounts of isophthalic acid or dimethyl isophthalate as starting materials for the copolymers, as this will result in the formation of prepolymers which are totally amorphous. Such prepolymers possess a particularly low glass transition temperature (the temperature at which the polymer properties change from those of a brittle solid to those of a rubber) and prepolymer particles will tend to stick together if solid-state polymerization is attempted at normally employed temperatures. Solid-state polymerization would have to be attempted at temperatures in the range of from about 140° C. to about 180° C., well below the polymer melting point, and at these low temperatures, the time periods for which polymerization would have to be conducted are so excessive as to be commercially impracticable.

The chain branching agents suitable for use in the present invention include one or more members selected from pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6; compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; compounds having the formula: ##STR11## wherein m is an integer from 1 to 6 and m' is an integer from 3 to 6; and compounds having the formula: ##STR12## wherein n is an integer from 3 to 4.

These chain branching agents may be formed in a variety of ways which are well known to those skilled in this art.

As indicated above, the chain branching agent incorporated in the present invention may be pentaerythritol. The branching agent may also be selected from compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6. Examples of such compounds include glycerol, sorbitol and hexane triol-1,2,6.

Also acceptable as branching agents are compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6. Examples of these compounds include trimethylolethane, trimethylolpropane and like compounds up to trimethylolhexane.

Another group of compounds which may serve as the chain branching agent used in the present invention are those having the formula: ##STR13## wherein m is an integer from 1 to 6 and m' is an integer from 3 to 6. Examples of such compounds include trimethylol benzene-1,3,5; tripropylol benzene-1,3,5; tributylol benzene-1,3,5; and similar compounds.

Finally, compounds having the formula: ##STR14## wherein n is an integer from 3 to 4 may serve as the chain branching agent used in the present invention. Examples of these compounds include trimesic acid, trimellitic acid, pyromellitic acid and prehnitic acid (1,2,3,4-benzene tetracarboxylic acid).

As previously indicated, the chain branching agent may be employed in the present invention, in amounts ranging from about 0.025 to about 1.5 mol percent, and preferably in amounts ranging from about 0.05 to about 1.0 mol percent. The term "mol percent" as used herein is the number of moles of chain branching agent over the number of combined moles of terephthalic acid and isophthalic acid present in the starting material expressed in percent.

Particularly preferred chain branching agents include pentaerythritol and trimethylolpropane.

The presence of a chain branching agent alone is insufficient for the purposes of this invention. In the absence of a chain terminating agent the degree of polymerization achieved during solid state polymerization may reach a level exceeding the critical gel point as defined by Flory. (P. J. Flory, "Principles of Polymer Chemistry", p. 350 ff., University Press, 1953, which is herein incorporated by reference). When this occurs, gel particles are formed which mar the appearance of a formed article. The polymer of the present invention is thus "gel-free", i.e., there are substantially no gels present in the polymer.

The chain terminating agents suitable for use in the present invention must have a boiling point of above about 200° C. and must also be stable at temperatures of up to about 305° C., to avoid excessive losses at operating temperatures. A compound which is considered "stable" at temperatures of up to about 305° C. is one which undergoes no significant decomposition including decarboxylation at that temperature. Preferably, the chain terminating agents suitable for the present invention have boiling points of above about 240° C.

Chain terminating agents suitable for use in the present invention include one or more members selected from aromatic monocarboxylic acids having the formula: ##STR15## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated, aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; monofunctional esters having the formula: ##STR16## where R³ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR17## wherein Y is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, alkoxy group containing from 1 to 10 carbons atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.q CH.sub.3-q COOH

where q is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁴ is a member selected from the group consisting of benzyl radicals and aliphatic radicals having from 1 to 18 carbon atoms; and which are the reaction product of an alcohol and a monocarboxylic acid; and difunctional esters having the formula: ##STR18## wherein R⁵ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR19## wherein Z is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.r CH.sub.3-r COOH

where r is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁶ is a member selected from the group consisting of saturated aliphatic radicals having from 2 to 18 carbon atoms and saturated alicyclic radicals having from 5 to 18 carbon atoms; and which are the reaction product of a diol and a monocarboxylic acid.

These chain terminating agents may be formed in a variety of ways which are well known to those skilled in this art.

As indicated above, the chain terminating agents incorporated in the present invention may be aromatic monocarboxylic acids having the formula: ##STR20## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group. Substitution may be in either one or all of the 3, 4, and 5 positions. Substitution in the 2 or 6 positions may be only in either of these two positions, and is limited to aliphatic radicals containing up to two carbon atoms or methoxy or ethoxy groups. Halogen and nitrogen substitution are also acceptable. The appended claims are meant to include these limitations with respect to acceptable substitution.

The chain terminating agent may also be a heterocyclic aromatic monocarboxylic acid. Examples of such acids include β-furoic acid and coumarillic acid.

Fused polycyclic aromatic monocarboxylic acids are acceptable as chain terminating agents. Examples of such acids include, e.g., those of the formulae: ##STR21##

Aryl aliphatic monocarboxylic acids such as phenylacetic and diphenylacetic acid are also acceptable.

Substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms may also be incorporated as the chain terminating agents of the present invention. Examples of such acids include caproic, heptylic, caprylic, pelargonic, capric, lauric, myristic, stearic and palmitic acids. Unsaturated acids include those such as oleic, linoleic and linolenic acids. Lower homologues with proper substitution which result in a sufficiently high boiling point, i.e., above about 200° C., are also contemplated.

Further contemplated are substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 atoms such as cyclohexane carboxylic acid, cyclopentane carboxylic acid and 2-Decalin® carboxylic acid. Decalin® is a registered trademark of E.I. duPont de Nemours & Co. for decahydronaphthalene.

Monoesters contemplated for incorporation as chain teminating agents include, e.g., benzyl benzoate, benzyl stearate, benzyl diphenyl acetate, ethylene glycol monobenzoate and ethylene glycol monostearate. Diesters include, e.g., ethylene glycol distearate and ethylene glycol dibenzoate. The diesters are esters of diols and not dicarboxylic acids. The acid constituents of the esters need not be limited to those previously mentioned chain terminating agents. The only criteria for selecting these constituents are that the boiling points be above about 200° C. preferably above about 240° C. and that the esters be stable at temperatures up to about 305° C.

The amount of chain terminating agent used is based on the number of equivalents of hydroxyl groups present in the particular amount of chain branching agent used. For example, 0.1 mole of trimethylolpropane contains 0.3 equivalents of hydroxyl groups. Generally, the amount of chain terminating agent used ranges from about 0.25 to about 10 equivalents based on the number of moles of chain branching agent used. Preferably, the amount ranges from 0.50 to about 5 equivalents on this basis. Preferred chain terminating agents include benzoic and stearic acids.

In another embodiment of the present invention, a single compound is used which acts both as a chain branching agent and a chain terminating agent. This compound is a polyfunctional ester having the formula: ##STR22## wherein a is an integer from 3 to 6. The polyfunctional ester has a boiling point of above about 200° C., preferably above about 240° C., is stable at temperatures of up to about 305° C., and may be made by a variety of methods well known to those skilled in this art. The ester is used in amounts from about 0.025 to about 1.5 mol percent, preferably from about 0.05 to 1.0 mol percent. The alcohol portion of the ester may include any of the alcohol chain branching agents previously discussed above. The acid moiety would include those acids defined above as acceptable chain terminating agents, as well as lower boiling (i.e., with a boiling point of about 200° C. or less) acids such as, e.g., formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid and isovaleric acid, or other lower boiling aromatic or alicyclic acids.

In some instances, the polyfunctional ester can only supply a maximum of one equivalent of chain terminating agent. In these instances, additional quantities of other chain terminating agents selected from those defined above may be used in amounts ranging from 0.1 to 5 equivalents based on the number of hydroxyl groups present in the particular amount of polyfunctional ester used. Preferred polyfunctional esters are trimethylolpropane tribenzoate and pentaerythritol tetrastearate.

In addition, mixtures of polyfunctional esters and polyfunctional alcohols as well as monocarboxylic acids can be used together in particular situations.

The chain branching agent and chain terminating agent or single compound acting as both are incorporated with a starting material which is selected from the group consisting of a terephthalic acid/isophthalic acid mixed monomer (mixed oligomers of terephthalic acid and ethylene glycol and isophthalic acid and ethylene gycol), a mixture of dimethylterephthalate and dimethylisophthalate, and a mixture of diesters of terephthalic acid and ethylene glycol and of isophthalic acid and ethylene gycol.

These starting materials may be made by a variety of ways known to those skilled in this art. For example, the mixed monomer may be prepared by reacting terephthalic acid and ethylene glycol [and separately isophthalic acid and ethylene glycol] (mole ratio of acid to ethylene glycol of 1:2) with choline (about 500-700 parts per million based on the weight of the acid) at a temperature of about 265° C. and under 50 psi nitrogen. Ethylene glycol and water are collected until an oligomeric material remains in the reaction vessel. The oligomer may subsequently be combined in appropriate amounts. Diesters of terephthalic acid and ethylene glycol and of isophthalic acid and ethylene glycol may be prepared, e.g., by the reaction of terephthalic acid and ethylene oxide [and separately isophthalic acid and ethylene oxide] (mol ratio=1:2.5) at a temperature of about 160° C., as a 10% solution in benzene in a Parr Bomb for about 3 hours. The terephthalic acid diester is commonly referred to as BHET (bishydroxyethylterephthalate) and the isophthalic acid diester is referred to as bishydroxyethylisophthalate. The diesters may be subsequently combined in appropriate amounts.

When a transesterification catalyst is needed, e.g., when dimethylterephthalate, dimethylisophthalate and ethylene glycol are selected as the starting materials, a weakly basic catalyst such as zinc acetate should be employed prior to the polymerization step. Such reactions are discussed in detail in Polymer Chemistry, Malcolm P. Stevens, Addison-Wesley Publishing Co., Inc., 1975, which is hereby incorporated by reference, and in particular at pages 251-254 thereof.

These materials are incorporated to form a prepolymer blend, which is heated, e.g., in an oil bath, to a temperature of about 180° C.

Into the heated prepolymer blend is incorporated any standard PET catalyst which does not interfere with the function of the chain branching and terminating agents. These catalysts are well known to those skilled in the art and include, e.g., antimony trioxide and antimony pentoxide. They may be incorporated in catalytically effective amounts, e.g., from about 300 ppm to about 1000 ppm based on the weight of copolymer made.

The temperature of the once modified prepolymer blend is increased to from about 200° C. to about 220° C. and the blend is again modified by the incorporation of a thermal stabilizing agent. Examples of such stabilizing agents include, e.g., polyphosphoric acid, triphenyl phosphite, tridecyl phosphite and trimethyl phosphite. The stabilizers may be incorporated in an amount effective to stabilize the prepolymer blend, e.g., ranging from about 25 to about 50 parts phosphorus per million parts polymer in the final product.

The chain branching and chain terminating agents may be conveniently incorporated, e.g., as ethylene glycol slurries. This method of incorporation is also convenient for the PET catalysts and thermal stabilizing agents.

After incorporation of the stabilizing agent, the temperature of the blend is increased to from about 260° C. to about 270° C., and a vacuum is applied to lower the pressure on the blend to less than about one millimeter of mercury over a period of less than about one hour. Preferably, the pressure is lowered to less than about 0.5 millimeters of mercury.

After achieving the necessary vacuum, the temperature of the blend is increased to from about 280° C. to about 290° C., and maintained at that temperature under vacuum for a period of at least about one hour, and more generally for a period of about one to about three hours, depending on the nature and amount of additives incorporated therein.

Subsequently, the prepolymer blend is either extruded into a quenching medium to form an amorphous strand of solid prepolymer or allowed to cool in situs to a temperature of about 25° C. to form a crystalline solid prepolymer mass. If extruded, the amorphous strand is cut into a plurality of amorphous prepolymer pellets which are slowly heated over a period of at least one hour to a temperature of above the glass transition temperature (T_(g) is equal to about 70° C. to 80° C.), preferably to a temperature of about 120° C., until the pellets are crystallized. An appropriate pellet size would be, e.g., a cylinder of about 1/16" in length and 1/32" in diameter.

Where the prepolymer mass is allowed to cool and solidify in situ, the solidified mass is subsequently milled to an appropriate size. E.g., the solidified mass may be pulverized in an Abbe Mill through a 0.25 in. hole screen. The material is sieved through a 9 mesh screen to remove fines. In either situation, sieving of the prepolymer particles to eliminate fines aids in the subsequent solid state polymerization step.

The final prepolymer has an inherent viscosity (I.V.) of from about 0.3 to about 1.0. After the PET prepolymer has been formed and pelletized or milled, the prepolymer particles are subjected to solid-state polymerization to form a high melt-strength, gel-free, branched copolymer suitable for extrusion blow molding applications. The copolymer is extremely shear sensitive, having a zero shear rate melt viscosity of from about 10⁵ to about 10⁶ at a temperature of from about 265° C. to about 300° C. poise and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise. The copolymer has an I.V. of above about 1.0 and greater than the I.V. of the prepolymer.

The prepolymer particles are solid-state polymerized by heating, e.g., in an oil bath, to a temperature of from about 185° C. to about 240° C., and preferably to about 210° C., over a period of at least about one hour. The presence of the isophthalic portion in the copolymer allows the polymerization step to be carried out at a temperature lower than the temperature employed for a corresponding pure PET prepolymer. The particles are solid-state polymerized under a vacuum of below about 0.5 millimeters of mercury, preferably from about 0.2 to about 0.5 millimeters of mercury, and down to as low as about 0.01 millimeters of mercury. The solid-state polymerization may be aided by constant or intermittent agitation, e.g., by tumbling or by the use of a sparge gas. The reaction may be carried out in an inert gas atmosphere.

Once heated, the prepolymer particles are maintained at the desired temperature and pressure for a period of at least about one hour. This period may extend for up to thirty or more hours, depending on the size of the prepolymer particles or chips. Polymerization is terminated when the desired I.V. (i.e., an I.V. of above about 1.0) is attained. This may vary, higher final I.V.'s being required for larger containers or bottles. Excessive contact of the polymer with oxygen or air during the solid-state polymerization process should be avoided, as such contact tends to degrade the polymer.

The modified and improved copolymers of the present invention may be extrusion blow molded on conventional extrusion blow molding equipment which is common in the container fabrication industry. Generally, blow molding machines comprises an extruder having a die gap at one end thereof and an openable mold positioned adjacent the die gap. The polymer is extruded through the die gap to form a parison, and the mold is closed about the parison. A gap is then introduced into the parison to expand it into the shape of the internal surface of the mold. Preferably, a blow pin is inserted into the parison and air is blown into the parison to cause it to expand and conform with the shape of the internal mold surface.

The extrusion blow molding process may be either continuous or intermittent. In the continuous system, a typical operation is as follows: The copolymer is fed to a hopper/dryer, and subsequently passed by way of a heated screw-type extruder to the die gap. Multiple sets of alternating die pairs close on parisons, which are formed at a rate of one about every ten seconds, and the desired containers are then blow molded to shape. Examples of end-use applications for the continuous process include the formation of shampoo and mouthwash bottles.

In the intermittent process, the extruder screw both rotates and translates, and a parison is formed in a period of about one second.

Temperatures in the heated screw extruder range from about 225° C. to about 295° C. Shear rates in the extruder for both the continuous and intermittent process reach a maximum of about 1000 seconds⁻¹. In the die, the maximum shear rates for the continuous process are about 500 seconds⁻¹, and up to about 50,000 seconds⁻¹ in the intermittent process.

The extrusion blow molding materials of the present invention may contain minor amounts of additional materials, such as fillers and plasticizers, so long as these additional materials do not detrimentally affect the physical properties of the extrusion blow molding materials.

The present invention is further illustrated by the following examples.

EXAMPLE I

A polyethylene iso/terephthalate prepolymer is prepared substantially in accordance with the procedure discussed hereinabove. About 0.3 weight percent of a chain branching agent consisting of trimethylolpropane (about 1.5 grams in 500 grams of mixed monomer) and about 1.6 equivalents of a chain terminating agent consisting of benzoic acid (6.55 grams) are incorporated into 500 grams of mixed monomer prepared employing as starting material terephthalic acid and isophthalic acid in a ratio of about 95:5 by weight. About 0.15 grams of an antimony trioxide catalyst are employed, and the catalyst is added as an ethylene glycol slurry. About 10 ml. of a polyphosphoric acid stabilizer solution are also employed. The stabilizer solution is taken from a bulk solution made with about 0.2 grams polyphosphoric acid in about 50 ml. of ethylene glycol. After processing for a period of over 2 hours, the prepolymer has an I.V. of between about 0.9 and about 1.0 (equal volume parts of trifluoroacetic acid and methylene chloride, 0.2 weight percent, 25° C.).

Once obtained, the prepolymer is solid-state polymerized in a manner substantially in accordance with the procedure discussed hereinabove. The solid-state polymerization is performed at a temperature of about 210° C. under a vacuum of about 0.05 millimeters of mercury for a period of about 3 hours. The copolymer obtained can be successfully processed by conventional, extrusion blow molding equipment to form a hollow container which is transparent throughout.

COMPARATIVE EXAMPLE A

A polyethylene iso/terephthalate prepolymer is prepared substantially in accordance with the procedure discussed hereinabove. About 0.3 weight percent of a chain branching agent consisting of trimethylolpropane (about 1.5 grams in 500 grams of mixed monomer) is used. No benzoic acid chain terminating agent is incorporated in this run. Again, 500 grams of mixed monomer prepared as in Example I is used as the starting material. About 0.15 grams of an antimony trioxide catalyst is employed, and is added as an ethylene glycol slurry. About 10 ml. of a polyphosphoric acid stabilizer solution is also employed. The stabilizer solution is taken from a bulk solution made with about 0.2 grams polyphosphoric acid in about 50 ml. of ethylene glycol. After processing for a period of about one to two hours, the prepolymer has an I.V. of about 0.9 (equal volume parts of trifluoroacetic acid and methylene chloride, 0.2 weight percent, 25° C.).

Once obtained, the prepolymer is solid-state polymerized in a manner substantially in accordance with the procedure discussed hereinabove. The solid-state polymerization is performed at a temperature of about 210° C. under a vacuum of about 0.05 millimeters of mercury for a period of about 3 hours. The copolymer obtained has a melt strength of about 1.2. An I.V. can not be determined due to the presence of an excessive amount of gels. The copolymer connot be successfully processed to form a hollow container by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment.

COMPARATIVE EXAMPLE B

A polyethylene terephthalate prepolymer is prepared and subsequently solid-state polymerized in a manner identical to Example I, except that no isophthalic acid is incorporated into the prepolymer blend (terephthalic acid is substituted for the isophthalic acid portion). The resulting PET polymer can be successfully processed to form a hollow container by conventional extrusion blow molding techniques using existing conventional, extrusion blow molding equipment. It is observed, however, that in several instances the thicker neck portions of the shaped containers are opaque.

EXAMPLE II

A polyethylene iso/terephthalate prepolymer is prepared substantially in accordance with the procedure discussed hereinabove. About 0.1 weight percent of a chain branching agent consisting of pentaerythritol (about 0.5 grams in 500 grams of mixed monomer) and about 2.0 equivalents of stearic acid (about 8.36 grams) are incorporated into 500 grams of mixed monomer, the mixed monomer being prepared employing as starting materials, terephthalic acid and isophthalic acid in a ratio of 90:10 by weight. About 0.15 grams of an antimony trioxide catalyst is employed, and is added as an ethylene glycol slurry. About 10 ml. of a polyphosphoric acid stabilizer solution is also employed. The stabilizer solution is taken from a bulk solution made with about 0.2 grams polyphosphoric acid in about 50 ml. of ethylene glycol. After processing for a period of over about 2.5 hours, the prepolymer has an I.V. of about 1.0 (equal volume parts of trifluoroacetic acid and methylene chloride, 0.2 weight percent, 25° C.).

Once obtained, the prepolymer is solid-state polymerized in a manner substantially in accordance with the procedure discussed hereinabove. The solid-state polymerization is performed at a temperature of about 210° C. under a vacuum of about 0.05 millimeters of mercury for a period of about two to three hours. The copolymer obtained has an I.V. of about 1.3. The copolymer can be successfully processed by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment to form a hollow container which is transparent throughout.

COMPARATIVE EXAMPLE C

A polyethylene iso/terephthalate prepolymer is prepared substantially in accordance with the procedure discussed hereinabove. About 0.1 weight percent of a chain branching agent consisting of pentaerythritol (about 0.5 grams in 500 grams of mixed monomer) is incorporated. A catalyst and stabilizer are employed in the same manner as in Example I. No stearic acid chain terminating agent is incorporated in this run. Again, 500 grams of mixed monomer prepared as in Example II was used as the starting material. After processing for a period of about two hours, the prepolymer has an I.V. of between about 0.9 and about 1.0 (equal volume parts of trifluoroacetic acid and methylene chloride, 0.2 weight percent, 25° C.).

Once obtained, the prepolymer is solid-state polymerized in a manner substantially in accordance with the procedure discussed hereinabove. The solid-state polymerization is performed at a temperature of about 210° C. under a vacuum of about 0.02-0.05 millimeters of mercury for a period of about 2.5 hours. An I.V. for the copolymer can not be determined due to the presence of an excessive amount of gels. The copolymer cannot be successfully processed to form a hollow container by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment.

COMPARATIVE EXAMPLE D

A polyethylene iso/terephthalate prepolymer is prepared substantially in accordance with the procedure discussed hereinabove. About 0.3 weight percent of a chain branching agent consisting of trimethylolpropane (about 1.5 grams in 500 grams of mixed monomer) and about 27.6 grams of a chain terminating agent consisting of poly(ethylene glycol monocetyl ether) (molecular weight of about 1000) are incorporated into 500 grams of mixed monomer, which is prepared as in Example II. A stabilizer and catalyst are employed in the same manner as in Example II.

Approximately one hour after maximum vacuum is obtained (0.5 millimeters of mercury), heavy gassing takes place indicating a substantial amount of decomposition of the ethylene glycol monocetyl ether. Thus, such a combination of chain branching agent and chain terminating agent would be totally unsuitable for the purposes of the present invention. The amount of chain terminating agent to be used corresponds to about 0.825 equivalents of chain terminating agent.

COMPARATIVE EXAMPLE E

A polyethylene terephthalate prepolymer is prepared and subsequently solid-state polymerized in a manner identical to Example II, except that no isophthalic acid is incorporated into the prepolymer blend (terephthalic acid is substituted for the isophthalic acid portion). The resulting PET polymer can be successfully processed to form a hollow container by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment. It is observed, however, that in several instances the thicker neck portions of the shaped containers are opaque.

EXAMPLE III

A mixed monomer is prepared in situ. The mixed monomer is prepared by charging terephthalic acid (about 147 pounds), isophthalic acid (about 26 pounds [the weight ratio of terephthalic acid to isophthalic acid is 85:15]) ethylene glycol (about 129.2 pounds) and choline (45 weight percent solution in ethylene glycol [about 10.89 grams]) to a reaction vessel. The temperature (Dowtherm) is about 265° C. About 50 psi of nitrogen is placed over the vessel. The reaction mixture is agitated. After about 30 minutes, distillation of water and ethylene glycol begins, and after the necessary amounts of water and ethylene glycol are removed, the agitation speed is reduced. About 298.8 grams of trimethylolpropane is introduced as an ethylene glycol slurry. Approximately five minutes after introduction of the trimethylolpropane, about 1633 grams of benzoic acid is introduced, also as an ethylene glycol slurry. Thus, trimethylolpropane is present in an amount of about 0.38 weight percent based on the combined amount of terephthalic acid isophthalic acids. Benzoic acid is present in an amount of about 2.0 equivalents of acid based on the number of moles of hydroxyl groups present in the trimethylolpropane charged. After a period of about five minutes (to allow for adequate mixing), about 36.4 grams of antimony oxide (about 400 parts per million based on the polymer) is added as an ethylene glycol slurry. This is followed by the addition of about 98 grams of a 10% solution of polyphosphoric acid in ethylene glycol. The temperature (Dowtherm) is set at about 285° C., and the vacuum is lowered to about 0.5 millimeters of mercury over a period of about 45 minutes. The reaction is allowed to proceed until the agitator pressure indicates that an I.V. of from about 0.6 to about 0.7 has been achieved. The material is extruded out under nitrogen pressure, quenched in water and subsequently dried.

Solid state polymerization is carried out after the prepolymer has been heated in air oven at a temperature of about 120° C. until crystallization occurs. The solid state polymerization is carried out in a Patterson Kelley Tubular Dryer at a temperature of about 190° C. and at a vacuum of from about 0.2 to about 0.5 millimeters of mercury for a period of about 24 hours. The modified copolymer has a zero shear rate melt viscosity at a temperature of about 280° C. of about 190,000 poise, and a melt viscosity at a shear rate of about 10³ seconds⁻¹ of about 5500 poise, at a temperature of about 280° C., and is capable of being processed at significantly lower temperatures.

About 100 pounds of the modified copolymer is fed to a hopper/dryer, and subsequently passed by way of a screw-type extruder (heated to a temperature of about 230° C.) to a die gap. The shear rate in the extruder is about 1000 seconds⁻¹ in the screw flight clearance (area between the narrow portion of the screw and the wall) and also in the die gap. The formation of a parison is continuous, taking about 5.4 seconds for completion of parison formation. A plurality of molds are arranged to be individually moved into a position surrounding and clamping each parison as formed. Once in position, the mold is moved out of the parison formation area and inflated by insertion of a blow pin into the parison. Air is introduced through the blow pin into the parison, causing it to expand and conform to the inner surfaces of the mold. As this procedure progresses, a new mold is moved into place in the parison formation area, surrounding and clamping the next parison to be formed. The modified copolymer of the present invention is processed in the blow molding apparatus with a minimum of torque development. A finished, extrusion blow molded container made in accordance with this invention is a tough material having a clear, smooth appearance and which is transparent throughout. It possesses a minimum of gels and a uniform wall thickness except in the neck portion thereof, where the wall is thicker than in the body of the container.

COMPARATIVE EXAMPLE F

A high molecular weight polyethylene iso/terephthalate copolymer is prepared in a manner similar to that set out in Example III, except that the chain branching agent and/or chain terminating agent are not incorporated. Such a product has a zero shear rate melt viscosity of about 110,000 poise at a temperature of about 280° C., and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature of about 280° C. of about 10,000 poise. Such a material lacks the shear sensitivity of the melt for blow molding applications, and requires extremely high processing temperatures (greater than 300° C.), with the extrusion blow molding machine operating very near to or in excess of the extruder torque limit.

COMPARATIVE EXAMPLE G

A polyethylene terephthalate prepolymer is prepared and subsequently solid-state polymerized in a manner identical to Example III, except that no isophthalic acid is incorporated into the prepolymer blend (about 173 pounds of terephthalic acid is charged to the reaction vessel). The resulting PET polymer can be successfully processed to form a hollow container by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment. It is observed, however, that in several instances the thicker neck portions of the shaped containers are opaque.

EXAMPLE IV

A polyethylene iso/terephthalate prepolymer is prepared substantially in accordance with the procedure discussed above in Example III. The trimethylolpropane chain branching agent and benzoic acid chain stopping agent are substituted for with about 656.3 grams of pentaerythritol tetrastearate. The prepolymer has an inherent viscosity of about 0.7 (0.2 weight percent in o-chlorophenol at 25° C.). Solid-state polymerization is carried out after the modified prepolymer has been heated in an air oven to a temperature of about 125° C. for several hours in order to crystallize the modified prepolymer.

The material is placed in a Patterson-Kelley Tubular Vacuum Dryer and heated to a temperature of from about 175° C. to about 190° C. for about three hours to obtain additional crystallization. (Such additional crystallization avoids the tendency of the polymer to stick to the walls of the Tubular Vacuum Dryer.) Solid-state polymerization is then carried out for a period of about twelve hours at a temperature of about 190° C. under a vacuum of from about 0.3 to about 0.5 millimeters of mercury. The copolymer has a melt strength of about 1.1 and an inherent viscosity of about 2.2 (0.2 weight percent in HFIP at 25° C.).

The copolymer may be blow molded to form a container which is transparent throughout in a manner similar to that discussed above in Example III.

EXAMPLE V

A polyethylene iso/terephthalate prepolymer is prepared by employing as a starting material a mixture of dimethyl terephthalate and dimethyl isophthalate (in a weight ratio of about 95:5). This mixture is added to a reactor vessel with ethylene glycol in a mole ratio of ethylene glycol to terephthalate mixture of about 2.2 to 1. The mixture is melted (e.g., at a temperature of about 150° C.). An ester interchange catalyst (e.g., zinc acetate) is incorporated in an amount sufficient to effect ester interchange. Reactor temperature is gradually increased to about 225°-230° C., and the ester interchange is initiated at about 150° C. and completed at about 180°-190° C. The temperature is maintained at about 225°-230° C. until the methanol formed as a by-product of the ester interchange reaction is completely removed. The reactor temperature is then lowered to about 180° C. and about 0.1 weight percent of a chain branching agent consisting of pentaerythritol (about 0.5 grams in 500 grams of esterified mixture) and about 2.0 equivalents of stearic acid (about 8.36 grams) are incorporated into 500 grams of esterified mixture. About 0.15 grams of an antimony trioxide catalyst is employed, and is added as an ethylene glycol slurry. About 10 ml. of a polyphosphoric acid stabilizer solution is also employed. The stabilizer solution is taken from a bulk solution made with about 0.2 grams polyphosphoric acid in about 50 ml. of ethylene glycol. After processing for a period of over about 2.5 hours, the prepolymer has an I.V. of about 1.0 (equal volume parts of trifluoroacetic acid and methylene chloride, 0.2 weight percent, 25° C.).

Once obtained, the prepolymer is solid-state polymerized in a manner substantially in accordance with Example I. The copolymer can be successfully processed by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment to form a hollow container which is transparent throughout.

COMPARATIVE EXAMPLE H

A polyethylene iso/terephthalate prepolymer is prepared in a manner identical to Example I, except that the mixed monomer is prepared employing as starting materials terephthalic acid and isophthalic acid in a ratio of 75:25 by weight. The prepolymer is completely amorphous and therefore is not solid-state polymerized.

COMPARATIVE EXAMPLE J

A polyethylene iso/terephthalate prepolymer is prepared in a manner identical to Example V, except that the mixed monomer is prepared employing as starting materials dimethyl terephthalate and dimethyl isophthalate in equal parts by weight (i.e., a weight ratio of 50:50). The prepolymer is completely amorphous and therefore is not solid-state polymerized.

The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention. 

We claim:
 1. A high melt-strength, gel-free, branched polyethylene iso/terephthalate copolymer, capable of being extrusion blow molded to form shaped articles which are transparent throughout, comprising a polyethylene iso/terephthalate copolymer which has been modified by the incorporation of: (a) from about 0.025 to about 1.5 mol percent of chain branching agent selected from one or more members of the group consisting of pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6; compounds having the formula:

    R.sup.2 -(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; compounds having the formula: ##STR23## where m is an integer from 1 to 6 and m' is an integer from 3 to 6; and compounds having the formula: ##STR24## wherein n is an integer from 3 to 4; and (b) from about 0.25 to about 10 equivalents based on the number of moles of (a) of a chain terminating agent, the chain terminating agent having a boiling point of above about 200° C. and being stable at temperatures of up to about 305° C. and selected from one or more members of the group consisting of aromatic monocarboxylic acids having the formula: ##STR25## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; monofunctional esters having the formula: ##STR26## wherein R³ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR27## wherein Y is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.q CH.sub.3-q COOH

where q is an integer from 1 to 3; substituted and unsubstituted aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁴ is a member selected from the group consisting of benzyl radicals and aliphatic radicals having from 1 to 18 carbon atoms; and which are the reaction product of an alcohol and a monocarboxylic acid; and difunctional esters having the formula: ##STR28## wherein R⁵ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR29## wherein Z is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.r CH.sub.3-r COOH

wherein r is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁶ is a member selected from the group consisting of saturated aliphatic radicals having from 2 to 18 carbon atoms and saturated alicyclic radicals having from 5 to 18 carbon atoms; and which are the reaction product of a diol and a monocarboxylic acid; the modified polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise.
 2. A high melt-strength, gel-free, branched polyethylene iso/terephthalate copolymer, capable of being extrusion blow molded to form shaped articles which are transparent throughout, comprising polyethylene iso/terephthalate copolymer which has been modified by the incorporation of from about 0.025 to about 1.5 mole percent of a polyfunctional ester having a boiling point of above about 200° C. and being stable at temperatures of up to about 305° C. and having the formula: ##STR30## wherein a is an integer from 3 to 6, which is derived by the reaction of a chain branching agent selected from the group consisting of pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6; compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; and compounds having the formula: ##STR31## wherein m is an integer from 1 to 6 and m' is an integer from 3 to 6 with a chain terminating agent selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR32## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; the modified polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise.
 3. The modified high melt-strength, gel-free, branched polyethylene iso/terephthalate copolymer of claim 2 which has been additionally modified by the incorporation of from about 0.1 to about 5 equivalents based on the number of equivalents of hydroxyl groups present in the particular amount of polyfunctional ester used of one or more chain terminating agents having a boiling point of above about 200° C. and being stable at temperatures of up to about 305° C. and selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR33## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 atoms; monofunctional esters having the formula: ##STR34## wherein R³ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR35## wherein Y is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.q CH.sub.3-q COOH

where q is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁴ is a member selected from the group consisting of benzyl radicals and aliphatic radicals having from 1 to 18 carbon atoms; and which are the reaction product of an alcohol and a monocarboxylic acid; and difunctional esters having the formula ##STR36## wherein R⁵ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR37## wherein Z is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.r CH.sub.3-r COOH

where r is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and where R⁶ is a member selected from the group consisting of saturated aliphatic radicals having from 2 to 18 carbon atoms and saturated alicyclic radicals having from 5 to 18 carbon atoms; and which are the reaction product of a diol and a monocarboxylic acid.
 4. The copolymer according to claim 1 wherein the chain branching agent is pentaerythritol and is incorporated in an amount from about 0.05 to about 1.0 mol percent; and wherein the chain terminating agent is stearic acid and is incorporated in an amount from about 0.5 to about 5 equivalents based on the number of moles of pentaerythritol.
 5. The copolymer according to claim 1 wherein the chain branching agent is trimethylolpropane and is incorporated in an amount from about 0.05 to about 1.0 mol percent; and wherein the chain terminating agent is benzoic acid and is incorporated in an amount from about 0.5 to about 5 equivalents based on the number of moles of trimethylolpropane.
 6. The copolymer according to claim 1 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units in the copolymer is about 85:15 or above.
 7. The copolymer according to claim 1 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units ranges from about 85:15 to about 95:5.
 8. The copolymer according to claim 2 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units in the copolymer is about 85:15 or above.
 9. The copolymer according to claim 2 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units ranges from about 85:15 to about 95:5.
 10. The copolymer according to claim 3 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units is about 85:15 or above.
 11. The copolymer according to claim 4 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units ranges from about 85:15 to about 95:5.
 12. The copolymer according to claim 5 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units ranges from about 85:15 to about 95:5.
 13. A process for preparing a high melt-strength, gel-free branched polyethylene iso-terephthalate copolymer capable of being extrusion blow molded to form shaped articles which are transparent throughout, comprising;(a) forming a polyethylene iso/terephthalate prepolymer which has been modified by the incorporation of: (a) from about 0.025 to about 1.5 mol percent of chain branching agent selected from one or more members of the group consisting of pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6; compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; compounds having the formula: ##STR38## where m is an integer from 1 to 6 and m' is an integer from 3 to 6; and compounds having the formula: ##STR39## wherein n is an integer from 3 to 4; and (b) from about 0.25 to about 10 equivalents based on the number of moles of (a) of a chain terminating agent, the chain terminating agent having a boiling point of above about 200° C. and selected from one or more members of the group consisting of aromatic monocarboxylic acids having the formula: ##STR40## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (CH.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; monofunctional esters having the formula: ##STR41## wherein R³ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR42## wherein Y is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.q CH.sub.3-q COOH

where q is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and where in R⁴ is a member selected from the group consisting of benzyl radicals and aliphatic radicals having from 1 to 18 carbon atoms, and which are the reaction product of an alcohol and a monocarboxylic acid; and difunctional esters having the formula: ##STR43## wherein R is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR44## wherein Z is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.r CH.sub.3-r COOH

where r is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁶ is a member selected from the group consisting of saturated aliphatic radicals having from 2 to 18 carbon atoms and saturated alicyclic radicals having from 5 to 18 carbon atoms; and which are the reaction product of a diol and a monocarboxylic acid; the prepolymer having an inherent viscosity of from about 0.3 to about 1.0; (b) subsequently solid-state polymerizing the modified prepolymer to form a polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise, a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise, and an inherent viscosity greater than the inherent viscosity of the prepolymer.
 14. A process for preparing a high melt-strength, gel-free, branched polyethylene iso/terephthalate cooplymer, capable of being extrusion blow molded to form shaped articles which are transparent throughout, comprising:(a) forming a polyethylene iso/terephthalate prepolymer which has been modified by the incorporation of from about 0.025 to about 1.5 mol percent of a polyfunctional ester having a boiling point of above about 200° C. and being stable at temperatures of up to about 305° C. and having the formula: ##STR45## wherein a is an integer from 3 to 6; and which is derived by the reaction of a chain branching agent selected from the group consisting of pentaerythritol; compounds having the formula:

    R.sup.1 (OH).sub.k

wherein R¹ is a saturated aliphatic hydrocarbon radical containing from 3 to 6 carbon atoms and k is an integer from 3 to 6; compounds having the formula:

    R.sup.2 --(CH.sub.2 OH).sub.l

wherein R² is a saturated aliphatic hydrocarbon radical containing from 1 to 6 carbon atoms and wherein l is an integer from 3 to 6; and compounds having the formula: ##STR46## wherein m is an integer from 1 to 6 and m' is an integer from 3 to 6 with a chain terminating agent selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR47## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; the prepolymer having an inherent viscosity of from about 0.3 to about 1.0; (b) subsequently solid-state polymerizing the modified prepolymer to form a modified polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise, a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature in the range of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise and an inherent viscosity greater than the inherent viscosity of the prepolymer.
 15. The process for preparing a high melt-strength, gel-free branched polyethylene iso/terephthalate copolymer according to claim 14 which includes the step of additionally modifying the prepolymer by the incorporation of from about 0.1 to about 5 equivalents based on the number of equivalents of hydroxyl groups present in the particular amount of polyfunctional ester used of one or more chain terminating agents having a boiling point of above about 200° C. and being stable at temperatures of up to about 305° C. and selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR48## wherein X is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the forumla:

    (C.sub.6 H.sub.5).sub.p CH.sub.3-p COOH

where p is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 atoms; monofunctional esters having the formula: ##STR49## wherein R³ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR50## wherein Y is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen, and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.q CH.sub.3-q COOH

where q is an integer from 1 to 3; substituted and unsubstituted saturated and unsaturated aliphatic monocarboxylic acids having from 6 to 30 carbon atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁴ is a member selected from the group consisting of benzyl radicals and aliphatic radicals having from 1 to 18 carbon atoms; and which are the reaction product of an alcohol and a monocarboxylic acid; and difunctional esters having the formula: ##STR51## wherein R⁵ is an organic radical derived from a member selected from the group consisting of aromatic monocarboxylic acids having the formula: ##STR52## wherein Z is a member selected from the group consisting of hydrogen, an aliphatic radical containing from 1 to 10 carbon atoms, an alkoxy group containing from 1 to 10 carbon atoms, an aryloxy group containing from 1 to 10 carbon atoms, a halogen and a nitro group; heterocyclic aromatic monocarboxylic acids; fused polycyclic aromatic monocarboxylic acids; aryl aliphatic monocarboxylic acids having the formula:

    (C.sub.6 H.sub.5).sub.r CH.sub.3-r COOH

where r is an integer from 1 to 3; substituted and unsubstituted, saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 30 atoms; and substituted and unsubstituted alicyclic carboxylic acids having from 6 to 30 carbon atoms; and wherein R⁶ is a member selected from the group consisting of saturated aliphatic radicals having from 2 to 18 carbon atoms and saturated alicyclic radicals having from 5 to 18 carbon atoms; and which are the reaction product of a diol and a monocarboxylic acid.
 16. The process according to claim 13 wherein the prepolymer is formed by (a) incorporating the chain branching agent and the chain terminating agent with a starting material selected from the group consisting of mixtures of oligomers of terephthalic acid and ethylene glycol and isophthalic acid and ethylene glycol, mixtures of dimethylterephthalate and dimethylisophthalate, and mixtures of bishydroxyethylterephthalate and bishydroxyethylisophthalate to form a prepolymer blend; (b) heating the prepolymer blend to a temperature of about 180° C.; (c) modifying the prepolymer blend by incorporating a catalytically effective amount a catalyst capable of effecting the formulation of a polyethylene iso/terephthalate copolymer; (d) increasing the temperature of the once-modified prepolymer blend to from about 200° C. to about 220° C.; (e) again modifying the prepolymer blend by incorporating a thermal stabilizing agent in an amount effective to stabilize the prepolymer blend; (f) increasing the temperature of the twice-modified prepolymer blend to from about 260° C. to about 270° C.; (g) lowering the pressure on the twice-modified prepolymer blend to less than about one millimeter of mercury over a period of less than about one hour (h) increasing the temperature of the prepolymer blend to from about 280° C. to about 290° C.; (i) maintaining the prepolymer blend at a temperature of from about 280° C. to about 290° C. and at a pressure of less than about one millimeter of mercury for a period of at least one hour; (j) extruding the prepolymer blend into a quenching medium to form an amorphous strand of solid prepolymer; (k) cutting the amorphous strand of prepolymer into a plurality of amorphous pellets; and (l) increasing the temperature of the amorphous prepolymer pellets to a temperature above the glass transition temperature thereof over a period of at least about one hour until the amorphous prepolymer pellets are crystallized.
 17. The process according to claim 13 wherein the prepolymer is formed by (a) incorporating the chain branching agent and the chain terminating agent with a starting material selected from the group consisting of mixtures of oligomers of terephthalic acid and ethylene glycol and isophthalic acid and ethylene glycol, mixtures of dimethylterephthalate and dimethylisophthalate, and mixtures of bishydroxyethylterephthalate and bishydroxyethylisophthalate, to form a prepolymer blend; (b) heating the prepolymer blend to a temperature of about 180° C.; (c) modifying the prepolymer blend by incorporating a catalytically effective amount a catalyst capable of effecting the formation of a polyethylene iso/terephthalate copolymer; (d) increasing the temperature of the once-modified prepolymer blend to from about 200° C. to about 220° C., (e) again modifying the prepolymer blend by incorporating a thermal stabilizing agent in an amount effective to stabilize the prepolymer blend; (f) increasing the temperature of the twice-modified prepolymer blend to from about 260° C. to about 270° C.; (g) lowering the pressure on the twice-modified prepolymer blend to less than about one millimeter of mercury over a period of less than about one hour; (h) increasing the temperature of the prepolymer blend to from about 280° C. to about 290° C.; (i) maintaining the prepolymer blend at a temperature of from about 280° C. to about 290° C. and at a pressure of less than about one millimeter of mercury for a period of at least about one hour; (j) allowing the prepolymer to cool in situ to a temperature of about 25° C. to form a crystalline solid prepolymer mass; and (k) milling in crystalline solid prepolymer mass.
 18. The process according to claim 13 wherein the chain branching agent and chain terminating agent are incorporated by blending as ethylene glycol slurries.
 19. The process according to claim 13 wherein the catalyst and thermal stabilizing agent are incorporated by blending as ethylene glycol slurries.
 20. The process according to claim 13 wherein the modified prepolymer is solid-state copolymerized by:(a) heating the prepolymer to a temperature of from about 185° C. to about 250° C. over a period of at least one hour; and (b) maintaining the prepolymer at said temperature and at a pressure of less than about 0.5 millimeters of mercury for a period of at least about one hour and until a polyethylene iso/terephthalate copolymer having an inherent viscosity greater than the inherent viscosity of the prepolymer is formed.
 21. The process according to claim 13 wherein the chain branching agent is trimethylolpropane and is incorporated in an amount from about 0.05 to about 1.0 mol percent; and wherein the chain terminating agent is benzoic acid and is incorporated in an amount from about 0.5 to about 5 equivalents based on the number of moles of trimethylolpropane.
 22. The process according to claim 13 wherein the chain branching agent is pentaerythritol and is incorporated in an amount from about 0.05 to about 1.0 mol percent; and wherein the chain terminating agent is stearic acid and is incorporated in an amount from about 0.5 to about 5 equivalents based on the number of moles of pentaerythritol.
 23. The process according to claim 13 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer is about 85:15 or above.
 24. The process according to claim 13 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer ranges from about 85:15 to about 95:5.
 25. The process according to claim 14 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer is about 85:15 or above.
 26. The process according to claim 14 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer ranges from about 85:15 to about 95:5.
 27. The process according to claim 15 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer is about 85:15 or above.
 28. The process according to claim 16 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer is about 85:15 or above.
 29. The process according to claim 16 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer ranges from about 85:15 to about 95:5.
 30. The process according to claim 17 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer is about 85:15 or above.
 31. The process according to claim 17 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer ranges from about 85:15 to about 95:5.
 32. The process according to claim 21 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer ranges from about 85:15 to about 95:5.
 33. The process according to claim 22 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units incorporated into the prepolymer ranges from about 85:15 to about 95:5.
 34. A process for blow molding a modified polyethylene iso/terephthalate copolymer into containers on a blow molding machine comprising an extruder having a die gap at one end thereof and an openable mold positioned adjacent the die gap comprising: (a) extruding the modified polyethylene iso/terephthalate copolymer through the die gap to form a parison, the modified polyethylene iso/terephthalate copolymer having a zero shear rate melt viscosity at a temperature in the range of from about 265° C. to about 300° C. of from about 10⁵ to about 10⁶ poise and a melt viscosity at a shear rate of about 10³ seconds⁻¹ and at a temperature of from about 265° C. to about 300° C. of from about 10³ to about 10⁴ poise; (b) closing the mold about the parison; and (c) introducing a gas into the parison to cause it to expand and conform with the shape of the internal mold surface, the containers thus formed being transparent throughout.
 35. The process according to claim 34 wherein the extrusion is carried out on a continuous basis and wherein parisons are formed at intervals of about ten seconds.
 36. The process according to claim 34 wherein the extrusion is carried out on an intermittent basis and wherein parisons are formed at intervals of about one second.
 37. The process according to claim 34 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units in the modified copolymer is about 85:15 or above.
 38. The process according to claim 34 wherein the weight ratio of terephthalic acid structural units to isophthalic acid structural units in the modified copolymer ranges from about 85:15 to about 95:5. 